Sulfate conjugation reactions are critical in the biotransformation of steroid hormones, neurotransmitters and several drugs. Sulfation reactions are catalyzed by cytosolic sulfotransferases (SULTs). SULT1 Al is a SULT isoform that catalyzes the conjugation of endogenous and exogenous estrogens as well drugs such as acetaminophen, minoxidil and 4-hydroxytamoxifen. SULTI El is a distinct SULT isoform that also sulfates estrogens. We have recently identified three common SULT1A1 alleles with different frequencies in Caucasian and African American populations. One of those alleles, SULTIA1 *2 was associated with significantly lower SULT1A1 activity in human tissues. The other allele, SULT1A1*3, is frequent in the African American population (23 percent) but not in Caucasians (1 percent). Biochemical analysis of the recombinant SULT1A1 *3 allozyme indicated that it had a ten-fold greater affinity for the sulfation cosubstrate, but the functional significance of this allele in human tissue has not yet been evaluated. This proposal seeks to examine the functional and clinical significance of those SULT1A1 polymorphisms, particularly in the context of SULT1 El redundancy. Purified recombinant SULT1 Al allozymes will be biochemically characterized with estrogen and antiestrogen substrates and compared with the biochemistry of another SULT isoform, SULT1 El. The association between the SULT1A1 *3 allele and the level of SULT1A1 activity and immunoreactive protein will be examined in platelets from African American individuals to determine genotype/phenotype correlation. Preliminary data suggested that the SULT1 Al *2 protein and/or mRNA was less stable than the SULT1 Al *1 enzyme. Therefore, the kinetics of allele-dependent SULT1 Al protein and mRNA synthesis and degradation will be assessed. Proliferative and transcriptional response to 17beta-estradiol, 2-methoxyestradiol and 4-hydroxytamoxifen will be studied and compared in MCF-7 cells stably expressing SULT1 El or SULT1A1 allozymes. Finally, the association between SULT1A1 alleles and specific breast cancer characteristics will be examined in a cohort of 600 Caucasian and African American women with breast cancer. These results will increase our understanding of the contribution of SULT1A1 pharmacogenetics to individual variation in response to estrogens and an important antiestrogen.